This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which uniformly scan a volumetric region for three dimensional imaging.
When a volumetric region or three dimensional object is ultrasonically scanned for three dimensional imaging, it is desirable to completely and adequately sample or scan the region or object so that the resultant three dimensional image faithfully and completely represents the volumetric or three dimensional object. A number of techniques have been proposed for ultrasonically scanning volumetric regions with the array transducer scanheads widely in use today for conventional two dimensional planar imaging. Since array scanheads operate by scanning an image plane, an image volume may be scanned by sweeping the scanhead""s image plane through the volumetric region of interest. U.S. Pat. Nos. 5,353,354 and 5,474,073 describe a linear sweep technique by which a sequence of parallel image planes are acquired as the scanhead is moved across the volumetric region and the image plane moves through the region being scanned. The acquired image data has the x and y coordinates of the scanhead""s image plane, and each spatially distinct plane defines a z coordinate. When the scanhead produces a sector scan, the volume swept will have a wedge shape. When the scanhead produces a linear scan of parallel scanlines in each plane, the volume swept will have a cubic or rectangular box shape.
A second volumetric scanning technique is to rock or fan the scanhead about a pivot point at the skin surface. A device which precisely produces this rocking motion is described in U.S. Pat. No. 5,487,388. Depending upon the shape of the two dimensional image plane of the scanhead (linear or sector), this fanning technique will sweep through a wedge or pyramidal shaped volume of the body.
Yet a third volumetric scanning technique is to rotate the scanhead about a pivot point. This technique will sweep through a cylindrical or conical volume of the body when the scanhead is rotated about the center of the image plane, depending upon whether the scan plane is linear or sector shaped. Both external and internally operating scanheads have been developed for performing this scanning. The article xe2x80x9cMultidimensional Ultrasonic Imaging for Cardiologyxe2x80x9d by McCann et al., published in the Proceedings of the IEEE, vol. 76, no. 9 (September 1988) at pages 1063-73 illustrates the rotational scan plane technique and describes an externally applied scanhead which scans the heart trans-thoracically. The scan plane is rotated by rotating a phased array transducer in angular increments with a stepper motor. The use of the motor enables uniform control of the angular increments; in an illustrated application the scan plane is stepped in increments of exactly 1.8xc2x0. The rotational volumetric scanning technique can also be performed internal to the body with a multiplane transesophageal echocardiography (TEE) probe as described in U.S. Pat. No. 5,181,514. Since a multiplane TEE probe inherently performs the function of rotating an array transducer about its center, successive scan planes can be acquired and stored as the array transducer is rotated and used to form a three dimensional image.
The McCann et al. article refers to a characteristic of rotationally acquired image data, which is the nonuniform distribution of the image data. The data density is relatively high around the axis of rotation and relatively low at the periphery of the scanned volume. McCann et al. deal with this nonuniformity by filling in the spaces between pixels at the periphery by hole filling, a process also referred to as interpolation, filtering or smoothing. As McCann et al. point out, this processing can be intensive: up to ten iterations of smoothing were required by McCann et al. to produce a pleasing image. Unmentioned by McCann et al. is the possibility that the periphery may be spatially undersampled, leading to artifacts and inaccuracies in the smoothed data. It would be desirable to reduce or eliminate this problem with a scanning technique that produced more homogeneous, uniformly sampled volumetric data.
In accordance with the principles of the present invention, a technique and apparatus are provided for uniformly scanning a volumetric region about a central axis. Scanning beams in proximity to the central axis are relatively widely separated while satisfying the desired criteria for spatial sampling density. Toward the periphery of the volume the scanning beams are relatively more closely spaced to better equalize the spatial sampling densities at the central and peripheral locations of the volumetric region. A scanhead for performing the inventive technique is described having an array transducer which transmits and receives beams that are more widely spaced toward the center of the array and more closely spaced toward the ends of the array. Mechanical or electronic rotation of the array produces more uniformly sampled volumetric image data.